This offer is part of something called the Future Free Plan Research Project, and it's only the third time RingPlus has offered it. To get in on the action, you need two things: a Twitter account and an inactive Sprint-branded phone.

Don't have a phone? You can buy a refurbished HTC Desire 510 from RingPlus for just $50 (plus shipping). Use it, or think of it as a placeholder: You can transfer your free plan to another model later on -- nice if your current phone is still under contract or you just want to buy something better than the Desire. (For example, there's a glut of used iPhones right now, so you could score one on the cheap -- as long as it's a Sprint version -- and bring it over to RingPlus.)

Update:Aaargh, the Desire 510 appears to be sold out already. RingPlus had indicated they had a generous supply of the phones, so I'm checking to see what happened.

Whichever path you choose -- current phone or $50 HTC -- you'll need to tweet this very post (using the icon up top) and include #ringplus. You'll also need to link your Twitter account to RingPlus as part of the account-signup process.

You'll also need to keep a credit card on file with RingPlus to accommodate any overages, and after the first month, the company will charge that card $5 to keep as a top-up balance. This isn't $5 every month, mind you, just a one-time charge so that funds are available in case you exceed your monthly allotment. (RingPlus charges a mere 2 cents per extra minute, message and megabyte.)

There has to be some other catch, right? Actually, there are two. First, although you get 1,000 free text messages, you get zero free MMS (aka picture/video) messages. Those cost 4 cents each. Ah, but iPhone users who exchange media messages with other iOS users are off the hook, as Apple's Messages app relies on data, not MMS proper. Android users have MMS-free options as well in the form of data-powered messaging apps like WhatsApp.

Second, although RingPlus provides real cell service, and not VoIP calls, you do have to listen to RingPlus Radio -- "music, news, deals, surveys and other audio we play during your ringback tone space" -- when you place outgoing calls. I haven't had the opportunity to try RingPlus yet, so I don't know how intrusive this is.

I do know there's no activation fee, though if you're planning to port an existing number, there's a $7.99 charge. Also, domestic roaming costs 14 cents per minute and 55 cents per megabyte if you venture outside Sprint's "native" coverage areas. (I can't find a coverage map on the RingPlus site to indicate where those areas are. If you're rural, you should do some investigating.)

Ready to bite? If you already have a phone, just head to the RingPlus Plans section, choose the Future Phase 3 option, then create a new account. With that done, you'll need to link to your Twitter account, after which you can register your phone.

Of course, I'm also eager to hear your comments about this deal, especially if you're already familiar with RingPlus.

Bonus deal: Need a mobile charger that can also charge your laptop? ChargeTech makes just such a product, but usually it's on the pricey side. Not today: You can get a ChargeTech Portable Power Outlet for $59.95 shipped. That's for the 12,000mAh version; the bigger models are on sale as well ($99.95 and $119.95, respectively, for 18,000mAh and 24,000mAh).